kafka 的心跳是 kafka consumer 和 broker 之间的健康检查，只有当 broker coordinator 正常时，consumer 才会发送心跳。
consumer 和 rebalance 相关的 2 个配置参数：
参数名 --> MemberMetadata 字段
session.timeout.ms --> MemberMetadata.sessionTimeoutMs
max.poll.interval.ms --> MemberMetadata.rebalanceTimeoutMs
broker 端，sessionTimeoutMs 参数
broker 处理心跳的逻辑在 GroupCoordinator 类中：如果心跳超期， broker coordinator 会把消费者从 group 中移除，并触发 rebalance。
private def completeAndScheduleNextHeartbeatExpiration(group: GroupMetadata, member: MemberMetadata) {
// complete current heartbeat expectation
member.latestHeartbeat = time.milliseconds()
val memberKey = MemberKey(member.groupId, member.memberId)
heartbeatPurgatory.checkAndComplete(memberKey)
// reschedule the next heartbeat expiration deadline
// 计算心跳截止时刻
val newHeartbeatDeadline = member.latestHeartbeat + member.sessionTimeoutMs
val delayedHeartbeat = new DelayedHeartbeat(this, group, member, newHeartbeatDeadline, member.sessionTimeoutMs)
heartbeatPurgatory.tryCompleteElseWatch(delayedHeartbeat, Seq(memberKey))
}
// 心跳过期
def onExpireHeartbeat(group: GroupMetadata, member: MemberMetadata, heartbeatDeadline: Long) {
group.inLock {
if (!shouldKeepMemberAlive(member, heartbeatDeadline)) {
info(s"Member ${member.memberId} in group ${group.groupId} has failed, removing it from the group")
removeMemberAndUpdateGroup(group, member)
}
}
}
private def shouldKeepMemberAlive(member: MemberMetadata, heartbeatDeadline: Long) =
member.awaitingJoinCallback != null ||
member.awaitingSyncCallback != null ||
member.latestHeartbeat + member.sessionTimeoutMs > heartbeatDeadline
consumer 端：sessionTimeoutMs，rebalanceTimeoutMs 参数
如果客户端发现心跳超期，客户端会标记 coordinator 为不可用，并阻塞心跳线程；如果超过了 poll 消息的间隔超过了 rebalanceTimeoutMs，则 consumer 告知 broker 主动离开消费组，也会触发 rebalance
org.apache.kafka.clients.consumer.internals.AbstractCoordinator.HeartbeatThread 代码片段：
if (coordinatorUnknown()) {
if (findCoordinatorFuture != null || lookupCoordinator().failed())
// the immediate future check ensures that we backoff properly in the case that no
// brokers are available to connect to.
AbstractCoordinator.this.wait(retryBackoffMs);
} else if (heartbeat.sessionTimeoutExpired(now)) {
// the session timeout has expired without seeing a successful heartbeat, so we should
// probably make sure the coordinator is still healthy.
markCoordinatorUnknown();
} else if (heartbeat.pollTimeoutExpired(now)) {
// the poll timeout has expired, which means that the foreground thread has stalled
// in between calls to poll(), so we explicitly leave the group.
maybeLeaveGroup();
} else if (!heartbeat.shouldHeartbeat(now)) {
// poll again after waiting for the retry backoff in case the heartbeat failed or the
// coordinator disconnected
AbstractCoordinator.this.wait(retryBackoffMs);
} else {
heartbeat.sentHeartbeat(now);
sendHeartbeatRequest().addListener(new RequestFutureListener<Void>() {
@Override
public void onSuccess(Void value) {
synchronized (AbstractCoordinator.this) {
heartbeat.receiveHeartbeat(time.milliseconds());
}
}
@Override
public void onFailure(RuntimeException e) {
synchronized (AbstractCoordinator.this) {
if (e instanceof RebalanceInProgressException) {
// it is valid to continue heartbeating while the group is rebalancing. This
// ensures that the coordinator keeps the member in the group for as long
// as the duration of the rebalance timeout. If we stop sending heartbeats,
// however, then the session timeout may expire before we can rejoin.
heartbeat.receiveHeartbeat(time.milliseconds());
} else {
heartbeat.failHeartbeat();
// wake up the thread if it's sleeping to reschedule the heartbeat
AbstractCoordinator.this.notify();
}
}
}
});
}
/**
* A helper class for managing the heartbeat to the coordinator
*/
public final class Heartbeat {
private final long sessionTimeout;
private final long heartbeatInterval;
private final long maxPollInterval;
private final long retryBackoffMs; private volatile long lastHeartbeatSend; // volatile since it is read by metrics
private long lastHeartbeatReceive;
private long lastSessionReset;
private long lastPoll;
private boolean heartbeatFailed; public Heartbeat(long sessionTimeout,
long heartbeatInterval,
long maxPollInterval,
long retryBackoffMs) {
if (heartbeatInterval >= sessionTimeout)
throw new IllegalArgumentException("Heartbeat must be set lower than the session timeout"); this.sessionTimeout = sessionTimeout;
this.heartbeatInterval = heartbeatInterval;
this.maxPollInterval = maxPollInterval;
this.retryBackoffMs = retryBackoffMs;
} public void poll(long now) {
this.lastPoll = now;
} public void sentHeartbeat(long now) {
this.lastHeartbeatSend = now;
this.heartbeatFailed = false;
} public void failHeartbeat() {
this.heartbeatFailed = true;
} public void receiveHeartbeat(long now) {
this.lastHeartbeatReceive = now;
} public boolean shouldHeartbeat(long now) {
return timeToNextHeartbeat(now) == 0;
} public long lastHeartbeatSend() {
return this.lastHeartbeatSend;
} public long timeToNextHeartbeat(long now) {
long timeSinceLastHeartbeat = now - Math.max(lastHeartbeatSend, lastSessionReset);
final long delayToNextHeartbeat;
if (heartbeatFailed)
delayToNextHeartbeat = retryBackoffMs;
else
delayToNextHeartbeat = heartbeatInterval; if (timeSinceLastHeartbeat > delayToNextHeartbeat)
return 0;
else
return delayToNextHeartbeat - timeSinceLastHeartbeat;
} public boolean sessionTimeoutExpired(long now) {
return now - Math.max(lastSessionReset, lastHeartbeatReceive) > sessionTimeout;
} public long interval() {
return heartbeatInterval;
} public void resetTimeouts(long now) {
this.lastSessionReset = now;
this.lastPoll = now;
this.heartbeatFailed = false;
} public boolean pollTimeoutExpired(long now) {
return now - lastPoll > maxPollInterval;
} }
join group 的处理逻辑：kafka.coordinator.group.GroupCoordinator#onCompleteJoin